The Atomic Force Microscope (AFM) has been widely used to image surfaces at a resolution of fractions of a nanometer. The AFM typically comprises a cantilever having a sharp tip at one end that is used to scan the sample surface. When the tip is brought into proximity with the surface, forces between the tip and the sample lead to deflection of the cantilever. Measurement of this deflection is used to create an image of the surface. To alleviate the risk that the tip collides with the surface during scanning, which may cause damage, a feedback mechanism is typically employed to maintain a constant distance between the tip and the surface.
In order for the tip to be close enough to the surface for short-range forces to become detectable, while preventing the tip from sticking to the surface, the AFM cantilever is typically operated in “tapping” mode. This also avoids damage caused by dragging of the sharp tip across the surface in “contact” mode AFM. In tapping mode, the cantilever is oscillated such that during each cycle the tip comes in contact with the surface, and a restoring force provided by the cantilever detaches the tip from the surface. This is usually achieved by attaching a piezoelectric block to the other end of the cantilever, remote from the tip, which drives the cantilever assembly up and down at its resonant frequency, usually at frequencies of up to tens of kHz.
Tapping mode AFM generally relies upon differentiation of the phase of the tip from that of the driver. A typical AFM setup is shown in FIG. 1 and described in EP0397496A. A compliant cantilever 101 is attached at one end to a probe 102 having a sharp tip 103, and at its other end to a driver 104 for oscillating the probe in the z direction. Means (not shown) are provided for x-y scanning of the probe 102 across the sample surface 105. Deflection of the cantilever in the z direction is measured by optical interferometry. The output of a laser 106 is directed via a beam splitter 107, set at 45 degrees to the beam axis, and a lens 108 to a reflecting surface 109 provided on the probe end of the cantilever 101 and separately to a plane mirror 110. Light incident on the plane mirror 110 is reflected back to the beam splitter 107, and light incident on the reflecting surface 109 is reflected back to the beam splitter 107. The two reflected beams meet and interfere at the beam splitter 107 and the resultant fringe pattern is directed to a light detector 111 via lens 112. The electrical output of the light detector 111 is applied to one input of a phase detector 113 forming part of a phase locked loop 114. The other input to the phase detector 113 is taken from a loop voltage controlled oscillator (VCO) 115. The output from the VCO 115 is also applied to the driver 104. The control voltage for the VCO is developed from the output of the phase detector 113 by means of a loop filter 116.
The tip 103 attached to, or integral with, the end of the cantilever 101 is deemed to have an egregious atom which interacts with the Van der Waals forces of the atoms of the sample surface 105. The compliance of the cantilever 101 allows a phase discrepancy to exist between the driver 104 and the tip as a result of such interaction. The detection of this discrepancy in the output from loop filter 116 is used to control the height of the tip 103, via a z height controller, at a constant distance from the surface 105 from which the contour is derived in a raster scan to produce a topographic x,y image of the surface.
Tuning the oscillator frequency of the VCO 115 to the resonant frequency of the cantilever assembly, in its free/un-biased state, improves sensitivity and noise performance. However, the amplitude and frequency of the oscillation in tapping mode AFM are clearly constrained by the resonant frequency and mechanical properties of the cantilever, leading to limited scan speeds. Harmonic operation is theoretically possible to increase scan speed, but virtual world modelling suggests that distortion and twisting of the cantilever occurs easily in this mode, particularly where the tip is influenced by lateral forces.
While tapping mode AFM has worked well enough in air, the effect of a liquid environment, as for electrochemical and live cell studies, is to lower the resonant frequency of the cantilever by a factor of five or more, and to introduce cavitation and turbulence into the liquid environment near the surface. The time taken to scan even a limited area is extended considerably and thermal fluctuations cause drifts in laser alignment.
In an article by A. G. Onaran, et al, entitled “A new atomic force microscope probe with force sensing integrated readout and active tip” published in Review of Scientific Instruments 77, 023501 (2006) an alternative to the conventional cantilever probe is described. The probe described in this article comprises a sharp probe tip provided on a micromachined optically reflective membrane, The membrane is, in turn, mounted on a transparent substrate incorporating a diffraction grating resulting in an integrated phase-sensitive structure. Thus, probe tip displacement is monitored by illuminating the diffraction grating and monitoring the intensity of the reflected diffraction orders. The tip of the probe is moved by electrostatic forces applied to the membrane with the transparent substrate as a rigid actuator electrode.
A further alternative to the conventional cantilever probe is described in an article by Toshu An, et al, entitled “Atomically-resolved imaging by frequency-modulation atomic force microscopy using a quartz length-extension resonator” published in Applied Physics Letters 87, 133114 (2005). This article describes the use of a quartz rod as a resonator to which a probe tip is attached. Shifts in the resonance frequency of the probe are representative of the interactive force between the probe tip and the sample surface and thus by applying a small oscillation amplitude to the resonator, characteristics of the surface of the sample may be imaged.
There is therefore a need in the art for an improved SPM to address at least some of the drawbacks evident in AFM.